This invention relates to DC-to-DC converters and, in particular, to quasi-resonant converters using Insulated Gate Bipolar Transistors (IGBT).
Quasi-resonant switching converters are used to convert one level of DC voltage and current to another level of DC level and current. They provide a similar function for direct current conversion that transformers provide for AC current and voltage converters use power semiconductor devices that turn off and on at relatively high rates. DC output is modulated by the time that the power devices are either on or off. This time is typically referred to as the xe2x80x9cduty cyclexe2x80x9d. The devices can be rapidly switched on or off and the resulting output is applied to an output filter for smoothing out the variations and switching waveforms.
Modern switching converters frequently use techniques that were pioneered by Edward J. Miller in his U.S. Pat. No. 4,138,715 issued Feb. 6, 1979. In that patent the basic problem of switching losses in DC converters is solved by applying a tank circuit to the switch. When power semi-conductor devices are switched at full current or full voltage, it takes a significant amount of time for the device to return to zero current or zero volts. In that time, the device and the circuit can experience short transients with relatively high peaks. These transients represent a waste of power and also a source of unwanted electromagnetic interference. In addition, rapid switching at high currents and high voltages creates thermal stresses on the power semi-conductors and thereby limits their useful life.
IGBTs are popular switching devices. One of their benefits is that they provide very low resistance in their on state. In addition, they can be rapidly switched on and off using their MOS controlled gate. In general, their on resistance is substantially less than power MOSFETs and they can handle higher currents for similar size devices. However, one drawback of the IGBT is that it includes minority carriers. If there is a high voltage or a high current in the collector when the IGBT is switched off, those carriers must be removed. Unfortunately, the minority carriers decay by recombination over a period of time. In effect, the presence of minority carriers in the collector resembles the discharge of a capacitor. When there are a large number of minority carriers, they can generate a substantial amount of heat until they are removed from the collector. This feature of IGBTs has limited their usefulness in DC-to-DC converters.
The invention overcomes the defects of the prior art and solves the problem of the slow decay of minority carriers in IGBTs by providing a quasi-resonant circuit for an IGBT in a DC-to-DC converter. A resonant tank circuit is coupled to the collector of the IGBT. The resonant tank circuit generates a quasi-sinusoidal waveform for the collector voltage and for the emitter current. After the collector voltage falls below ground, the gate pulse is terminated and the IGBT turns off. Then, the minority carriers on the collector are swept away by the inductor of the tank circuit. Those minority carriers are stored in a capacitor that is coupled across the primary winding of the transformer. In this way, the minority carriers are rapidly removed from the collector thereby quickly discharging the IGBT and making it ready for its next cycle.